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An Aquatic View of the Complexities of Light

By Ian Ashdown. Presented June 7th, 1998 on #reefs IRC.

This talk is about aquarium lighting, with an emphasis on the word
"light." It is *not* about who makes the best products or how various
species of marine life respond to different luminous environments. An
important caveat: I owned a 100-gallon freshwater aquarium some 35 years
ago when most people thought of aquaria as two goldfish in a glass bowl. I
am *not* familiar with today's vastly more advanced aquarium
technology.

As a lighting research engineer, I am however very familiar with my
favorite topic: light. It is an enormous field of discourse, ranging from
the physiology of human vision to photobiology and quantum mechanics. I
cannot hope to cover the entire field in relation to aquarium lighting in a
short talk. Instead, I will present four brief topics and then attempt to
answer your questions.

Measuring Light

Light consists of photons, with each photon having a specific amount of
energy. We see these photons as having different colors; photons with
increasing amounts of energy appear to us as pure red, orange, yellow,
green and blue light. Beyond the visible spectrum is infrared light (low
energy) and ultraviolet light (high energy). Most light sources emit
photons with a wide range of energies. What we perceive as "white" light is
really a mixture of photons ranging from infrared to ultraviolet. The
proportion of photons with different energies determines the color we
perceive the light source to be.

Our eyes do not see all photons equally. They are most sensitive to
yellow-green light, and are less sensitive to red and blue light. Of
course, we do not see infrared or ultraviolet light at all. Here's a
problem: when we measure light with a light meter

(typically illuminance in units of footcandles or candela per square
meter), we are measuring according to the color sensitivity of the human
eye. Unfortunately, it is highly unlikely that marine life will have the
same sensitivity to different colors of light. Water absorbs light of
different colors, and the rate of absorption depends on the presence of
suspended silt, plant and animal material, and living organisms such as
plankton. For example, the transmittance of water per meter for selected
locations is:

Morrison Springs, Florida

Gulf of Mexico

Long Island Sound

Thames River (Connecticut)

Loc 1

Loc 2

Loc 3

Loc 4

Ultraviolet (400 nm)

92 %

56 %

38 %

0 %

Blue (450 nm)

95 %

83 %

40 %

1%

Blue-green (500 nm)

97 %

95 %

45 %

4 %

Green (550 nm)

95 %

94 %

48 %

5 %

Orange (600 nm)

83 %

80 %

45 %

6 %

Red (650 nm)

74 %

73 %

40 %

12 %

Infrared (700 nm)

58 %

55 %

33 %

13 %

Remember, this is per meter of water. If you are trying to simulate an
environment at a depth of 4 meters, for example, and the transmittance is
74 % per meter for a given color, the amount of light reaching this depth
from the surface will be 0.74 * 0.74 * 0.74 * 0.74 = 30 percent. For a
transmittance of 97 % per meter, it is 86 percent. The spectral
distribution of light, even in the same geographical location, clearly
varies with depth.

My point is that marine species that occur at a given depth will mostly
likely have evolved their visual systems to take maximum advantage of the
available light. If you are measuring light with the intent of simulating a
species' natural enviornment as closely as possible, you should consider
the spectral distribution of the light. Measuring light according to our
visual system's color sensitivity may not be appropriate. To do this
properly is admittedly a problem. You need to know the spectral
transmittance of the water for the species' natural environment (a good
excuse to visit the Great Barrier Reef), which requires a fairly expensive
spectroradiometer (and a waterproof one at that). You also need to know the
spectral distribution of your light sources, which may not be readily
available from the lamp manufacturer.

These, however, are resolvable problems, especially in these days of
worldwide dissemination of data on the Web. The question is whether the
problem itself is a true concern or a theoretical exercise. (As an aside,
lamps for horticultural purposes are designed to produce most of
their light in the orange and red portions of the visible spectrum. Various
plant pigments involved in photosynthesis and plant growth are quite
sensitive to the spectral distribution of light in their environment. The
same is likely true of symbiotic marine algae in corals.)

Another issue regards underwater light measurements, particularly in
environments with light-colored sandy bottoms. Skiers and high- altitude
mountain climbers know all too well that the ultraviolet light reflected
from snow-covered slopes can cause sunburn almost as quickly as direct
sunlight. (Imagine being sunburned on the inside of your nostrils -- it
happens.) Relating this to keeping deep-water fish in an aquarium brings up
the same issue: the light reflected from the bottom of the aquarium is not
measured by a light meter facing the water surface.

Seeing Colors

We may think we can distinguish millions of colors, but we really can't.
Instead, the retinae of our eyes have color-sensitive "photoreceptors" that
can only distinguish three overlapping ranges of colors - red, green and
blue. What we see as a particular color are really electrical signals to
the brain from these photoreceptors. The color we perceive is due to the
relative strengths of these signals.

This leads to an interesting physiological effect and a possible problem
for aquarium lighting. Sunlight consists of photons with a continuous range
of energies from infrared through visible to ultraviolet light. We perceive
the color of sunlight as "white". However, because the human eye is
sensitive to only three ranges of colors, we may also perceive a mixture of
three specific colors - red, green, and blue - as being the same color of
white.

Lamp manufacturers take advantage of this effect by producing
fluorescent and high-intensity discharge (HID) lamps that emit light only
in narrow bands of the spectrum. We may see the emitted light as being
white, but it is not in any way equivalent to sunlight. Is this important?
It depends on how your aquatic friends perceive and respond to light. Some
species of shrimp have been shown to perceive ten different ranges of
colors (compared to our three). The evolutionary reasons for this
capability are unclear, but it is likely that it offers the shrimp benefits
in identifying food and fellow membera of their species. If you attempt to
recreate their environment using fluorescent or HID lamps, they may be
unable to see some colors. (Imagine living your life in an environment that
has only blue light.)

As an aside on this issue, lamp manufacturers often rate their lamps
with a Color Rendering Index (CRI). This index is a measure of how well the
lamp will render colors of various materials in comparison to daylight
(which is a mixture of direct sunlight and the clear blue sky). A
low-pressure sodium vapor streetlight, for example, renders most colors
very poorly and so has a low CRI. A lamp with a high CRI will allow us to
perceive most colors such as they would appear in daylight. Again however,
what we see in terms of colors is not necessarily what your aquatic friends
will see.

Sunburnt Starfish?

Simulating the environment of a shallow water tropical reef can be
challenging. The average illuminance of our homes and offices is on the
order of 100 to 500 candela per square meter (10 to 50 foot- candles).
Direct sunlight, on the other hand, is on the order of 10,000 candela per
square meter (1,000 foot-candles). That is a lotof light.

One obvious solution is to use high-intensity discharge (HID) lamps. As
long as you don't inadvertently cook your aquatic friends with an excess of
infrared energy, you should be fine. Right? Not necessarily. HID and
fluorescent lamps produce most of their light in the ultraviolet region of
the spectrum. The lamps bulbs are typically coated with rare earth
phosphors that absorb ultraviolet light and re-emit it as visible light.
The problem here is that the process is not 100-percent efficient.

Many HID and fluorescent lamps emit a proportional amount of ultraviolet
light well in excess of that found in natural daylight.

The water will absorb some of this excess ultraviolet light, but clear
water still transmits up to 90 percent of UV light per meter. In humans,
excessive exposure to ultraviolet light produces sunburn, skin cancer,
cataracts and and morphologic alterations of the skin, including wrinkling,
altered pigmentation, and thickening. It is likely that similar effects
occur in aquatic animals and plants. (Amphibians appear to be particularly
sensitive to excess amounts of ultraviolet light, especially in the egg and
larvalstages.)

The real problem is that most lamp manufacturers do not publish any
information on the relative proportion of ultraviolet light emitted by
their lamps. As one example, the amount of UV produced by two otherwise
identical compact fluorescent lamps was determined to differ by a factor of
four. Lighting manufacturers often use UV-inhibiting acrylics in their
fluorescent lamp diffusers, but this may not be a viable solution when the
lamps are producing 10 to 20 the amount of light normally found in
architectural light fixtures.

To address this problem requires an ultraviolet irradiance meter.
Unfortunately, the energies for ultraviolet photons varies considerably,
with wavelengths ranging from 257 to 453 nm. The ultraviolet spectrum is
divided into several regions according to wavelength, and the biological
effects of photons in these different regions varies. The problem is not a
simple one by any means.

Getting Technical With Microeinsteins

Poor Albert Einstein! Horticulturalists, looking for some means of
quantifying the effect of light from various sources (daylight, xenon
lamps, and metal halide lamps designed for greenhouses), named a unit of
light measurement after him - the einstein. Unfortunately, this is a *lot*
of light, so the units of measurement for real world applications is the
microeinstein. Marine biologists use the same unit of measurement when
referring to the response of symbiotic marine algae in coral.

The key concept is the absorption spectra of chlorophyll, that
wonderfully complex molecule that makes our world turn. (No chlorophyll, no
oxygen, no animals, no humans). It has a significant absorption range
between 550 and 720 nm, with peak absorption at 700 nm. Various pigments
alter the actual absorption characteristics of different plants, so
horticulturalists required some standardized means of measuring the
photosynthetic efficacy of polychromatic light. Some bright biologist
decided that if you plot the photon energy between 400 nm and 700 nm (the
limits of the visible spectrum, from deep blued to deep red), it grossly
approximates the spectral absorption characteristics of chlorophyll. This
we have photon (quantum) flux, in which the number of photons (rather than
their energy) per second per unit area is significant.

Plant biologists use what are called quantum meters to measure plant
irradiance, which is nothing more than a light with a color filter whose
spectral transmittance ensures that its response is directly proportional
to the number of photons rather than their energy. Needing a unit of
measurement, they turned to chemistry for the mole (6.23 x 10+23 atoms or
molecules in a mole). This gave them the microeinstein, or a micromole
(6.23 x 10+17) photons per second per square meter), of irradiance as a
measure of photosynthetically active radiation.

If you know the wavelength of monochromatic light, you know its energy
from Planck's law (see any college physics text), and so you can calculate
the number of photons per second per unit area for a given irradiance.
Integrate this over the region of 400 nm to 700 nm (the visible spectrum)
for a given light source and you have the plant irradiance value in
microeinsteins. It's simple once you understand the underlying concept.

As a rule of thumb: one watt/sec/m2 of sunlight or light from
horticultural lamps such as xenon or metal halide arc lamps is equivalent
to 4.5 to 5.0 microeinsteins. Similarly, one lux of visible light is
approximately equivalent to 0.01 to 0.02 microeinsteins. Your mileage may
vary for aquarium lighting, but you get the idea.

I doubt whether I have answered many questions with these discussions,
but this was not my intent. Rather, I wanted to offer a few topics for
discussion that may not appear in the usual literature. If you want to
discuss lamps and ballasts or light measurements, then by all means post
your questions. However If you have understood that there may be issues
with respect to aquarium lighting beyond that presented by the lamp
manufacturers (whose primary interest is in producing illumination for the
human visual system), then I will have accomplished my goals with this
talk.

- Ian Ashdown

OK, a microeinstein. It was in the talk, but it flew by too fast for
most people.
The basis of the "microeinstein" is a measurement of light irradiance.
That is, it is a meaure of how many photons illuminate an area (say one
square meter) per second.
In lighting, we measure this quantity as "candela per square meter", or
"footcandles".
It's really just a measure of how much light is illuminating a surface.
The problem is that we see light best when it is yellow-green in color.
We see red and blue light (at the ends of the visible spectrum) very
poorly - our eyes are not as sensitive to these colors.
Plants (presumably including marine algae) "see" light differently.
Photosynthesis is based on the ability of chlorophyll to absorb energy
from from light, and it does this most efficiently in the red region of
the spectrum.
The microeinstein takes this into account by preferentially measuring
orange and red light.

(The discussion has a more coherent explanation) -

Given that you say a microeinstien is designed for terrestral measurment
what can we use for underwater measuremtn so we can converse with each
other and compare apples to apples?

That's the problem - as Bretton Wade has informed me, there does not
appear to be an appropriate unit of measurement.
Certainly the Illuminating Engineering Society of North America (which
defines units of radiometric and photometric measurements) hasn't
considered the issue. - IA

Are dimmable MH balasts of any real benefit?

It depends on why you might want to dim the lamps. If you are trying
to provide a day-night cycle to maintain the biological rhythms of your
inhabitants, then yes.
However, this assumes that you have a programmable dimmer

Ian - you didn't use the term PAR - Photosynthetically Available
Radiation - How do you feel about that unit of measurement?

PAR is an equivalent for the microeinstein, or micromoles per second
per square meter.

What colors of spectrum seem to filter out quicker and is it a constant
rate?

The yellow region of the spectrum goes first.
The loss is geometric - if you have 74% transmittance per meter of
water, you have 74% afer one meter, 55% after two meters, 40 % after
three meters, and so on IA

Is there a chart we can refer to that shows the 'brightness' needed for
aquaria?

Ah, "brightness." Lighting engineering cringe when we hear that term -
it can mean too many things!
What we see when we look at a surface used to be called "photometric
brightness", but is now called "luminance".
Simply put, it is the average amount of light emitted or reflected from
a surface in the direction of the viewer.
Of course, this is not what you want. What you are asking for is
"irradiance", which is the average amount of light illuminating each
sqaure meter (or square foot) of a surface. It is measured in watts per
square meter.
The problem is that you are not illuminating a surface; you are
illuminating a *volume* of water.
The water absorbs the light according to the depth, so that the light
level at the bottom of an exceptional large aquarium may be significantly
less than at the surface - as little as 50 percent for 12 feet, for
example. For most of you (I am not an aquarium enthusiast), the aquarium
depth is more like one to two feet.
This means that the light loss is (assuming clear water) usually less
than 10 percent. In other words, all you really need to worry about is:
(a) how much light per unit area is illuminating your aquarium;
and (b) what is the spectral distribution of the lamps used for the
lighting system. Part (b) is the hard part, of course.

What about in turbulent water? Is there any way to approximate the
effect on the refractive index?

I'm not sure why you are interested in the refractive index of
light, but turbidity does not affect it. It is no different than trying
to see through smoke or fog -

I guess, as far as the amount that actually travels downward, versus
being redirected... or does it balance out?

for light travelling downwards, it has to enter the water
perpendicular to the surface. The refractive index of water has no
effect at this angle.

Will it ever be possible to ~fine tune~ lamps to favour zooanthelae
growth and not undesirable algae?

If by "fine tune" you mean adjust their spectral distribution, then
the best way is to use color filters.

Ian, is there any direct relationship between K ratings, nm and visually
perceptible colors?

K ratings - ask me another time by e-mail (byheart@direct.ca) - I have
to investigate the topic

Ian, could typical UV leakage from aquaria bulbs ever be an eye
hazard?

yes, for two reasons. First, looking directly at the lamps or their
reflections for extended periods of time may cause eye damage.
Second, high-powered HID lamps can generate small amount of ozone, which
is a hazardous gas.

Is an electric MH ballast really worth $100 more than a tar ballast? Is
there a significant difference in perfomance?

I assume you meant "electronic" ballast. These generally consume less
power (ie, emit less heat), and should last longer.
They also produce *much* less visible flicker. We can't usually see the
flicker, but I wonder what effect it has on animals with faster visual
system responses.
in general, yes. Better lifetime, less power loss, less flicker, cooler
operation.
The newer electronic ballasts (at least for fluorescent lamps)
incorporate "soft start" features that gradually increase the lamp
current.
This reduces the physical deterioration of the lamp electrodes and so
reduces premature lamp end blackening
However, I am not sure about HID lamp ballasts. These are newer entries
to the market, and I have yet to design
no, electronic ballasts generally do not affect the spectral
distribution of lamps under normal operating conditions.
again, a very general question with no single answer. See the
manufacturers' data for more information

Does manufacturer of the bulb matter as much as your ballast type when
predicting how long the bulb will live? Which dominates?

Most of the major lamp manufacturers have optimized their production
to achieve maximum lifetimes for the cost of the lamp.

They could make lamps that last forever (one incandescent lamp in a
baseball stadium has been burning continuously since the 1930s), but you
wouldn't want to pay for it, and the light output would be less than what
you would expect for the power used